Hot pressing with a temperature gradient



May 20, 1958 H. H. HAUSNER- HOT PRESSING WITH A TEMPERATURE GRADIENT Filed June 4, 1957 INVENTQR. HENRY H. HAUSNER 1 1 l I HIM/H1 H I 1 1 1/1 Henry H. Hausner, New York, N. Y., assignor, by mesne assignments, to the United States of America as represented by the United States sion Application June 4, 1957, Serial No. 663,554 3 Claims. (Cl. 75-226) Atomic Energy Commis- This invention relates to a method and apparatus for hot pressing of metal powders or powders of metal compounds; More particularly, it relates to an apparatus for hot pressing powders with a temperature gradient. The present application is a continuation in part of my copending application Serial Number 321,121, filed November 18, 1952, and now abandoned.

In the powder metallurgical art it has been observed that a longitudinal density gradient exists in compacted metal parts which have a length to width ratio on the order of approximately 2 to 1 or greater. This density gradient is directly attributable to the fact that there is a pressure gradient existing in every pressing operation in which, for the most part, the particles farthest away from the pressure ram are subjected to the lowest pressure. In order to avoid this gradient it has been proposed .in the prior art to press such slugs or other bulk-y objects in .a step-wise process in which each of the layers is individually pressed. For example, it has been suggested that the entire charge which is to be loaded into the die be broken up into several component parts in which the first component is charged into the die and then compressed with the aid of a ram while it is being heated with an external heating :coil. When the proper density has been obtained, the second component portion of the charge is introduced and the pressing and heating operation is repeated with the heat being directed onto that portion of the die containing the second component portion of the Charge. This operation is repeated until the entire body has been formed. The result is a body of fairly uniform density. This method, however, requires successive hot pressing operations on each of the portions and may tend to cause a laminar effect in the whole body portion. In none of the prior art has there been any method or apparatus which would permit relatively large bodies to be hot pressed in substantially one operation and still attain a substantially uniform high density throughout the entire body.

It is therefore an object of this invention to provide a method which will permit the pressing of bodies which have a length to width ratio approximately 2 to 1 or greater in a single pressing operation and still obtain a substantially uniform density throughout the body and in which the density gradient lies within very narrow limits throughout its longitudinal section.

It is a further object of this invention to provide a simple method for pressing long bodies in a single pressing operation.

Other objects will be in part apparent and in part pointed out in the description which follows.

It has been found that these objects and other advantages incidental to its application can be attained by hot pressing the part in a die whose temperature varies throughout its length in accordance with a predetermined pattern.

In the drawings which illustrate an embodiment of this invention:

Figure l is a diagrammatic view of a lengthy compressed body pressed under normal hot pressing conditions showing the densities normally obtained at different points through the length of the body.

Figure 2 is a diagrammatic view of a similarly shaped compressed body, pressed in accordance with this inventive concept showing the densities obtained through similar sections of the body.

Figure 3 is a vertical elevation, mainly in section, of a combination press with single acting ram and furnace with a mold therein filled with a powdered charge illustrating features of this invention.

Figure 4 .is a vertical elevation, mainly in section, similar to Figure 3 but illustrating this invention when a double-acting ram is used.

The variation in density which was obtained when relatively long bodies were pressed by powder metallurgical methods .in accordance with the prior art is graphically shown in Figure 1 wherein the top section which was closest to. themoving ram or pressure point has been shown as having a density of 99% whereas the next section immediately below having a comparable voltune exhibits a density of 97%. The third section of equal volume has a density of 95% and the fourth section a density of 92% and the lowermost section which is farthest away from the point at which the pressure was brought to bear has a density of The reason for obtaining this density gradient is most probably caused by the various frictional forces which occur during the pressing operation with the larger frictional forces making themselves felt at the points farthest removed from the pressure ram.

Figure 2 on the other hand graphically illustrates the factthat when practicing in accordance. with this invention the density of the body can be maintained constant throughout its entire length so that both the top and the bottom as well-as each of the comparable volume sections has the same density namely approximately 99% of the theoretical.

It has been stated above that control of the density can be obtained by holding the die in which the powder is to be compressed at a temperature that varies throughout its length in accordance with a predetermined pattern so that the die and the powder contained therein is heated to the highest temperature at those places farthest removed from the movable ram with the temperature gradually decreasing in-the direction toward the ram.

In those cases in which a single action die is used, as illustrated in the drawing in Figure 3, the die 10 is encased in a furnace 20' having heating elements 34 with the moldv or die lil'resting on a pressure plate 22. The powder from which the body is to be made is compressed with the aid of a plunger or 24. In order to maintain the mold at. a non-uniform temperature in accordance with this inventiveconcept, the. heating coil v30 as there shown in the drawings is so, arranged that the greatest amount of heat can be transmitted to the die 10 at the bottom thereof toward the pressure plates whereas the temperature varies gradually upwardly so as to give a definite temperature gradient to the die and the contained powder with the coolest portion being closest to the movable plunger or ram 24. Naturally this invention can also be used in the case of dies or molds in which the pressure is exerted from both ends namely, with a double action plunger or ram. In such case, as is illustrated in Figure 4 of the drawings, the coil 50 about the mold is so arranged that the greatest amount of heat is delivered to the die 60 at a point substantially half way between the moving plungers 52 and 54. In these cases the heating coil 50 is so arranged with respect to the end portions of the mold that there is a gradual temperature gradient outwardly from the center of the mold to either end.

When using a non-uniformly heated mold of the type herein described, it is possible to place the full powder charge within the mold, heat the mold to the desired temperature which, of course, brings the charge therein to a temperature which varies gradually fromend to end and is substantially hotter at the point or points farthest away from the moving plunger or ram which is to com press the parts. When this temperature has been established the pressure is applied by means of the ram or rams for the necessary time interval to compress the body to the desired extent. When this method is used, it is possible to obtain a body having a substantially uniform density throughout its length and one whose density will not vary in the direction of pressure.

It is believed that the success of this method is depend ent upon the fact that compressed powder is at its most fluid state in those parts of the die where the losses in pressure due to friction would be felt the most.

In practicing the invention with regard to specific metals, the following procedures are preferred: Using uranium powder of approximately to micron particle size, this powder is charged into a die having a diameter approximately one inch and a height of approximately 24 inches. The die is heated to develop a temperature gradient in the powder charge having a highest temperature of 640 at the point farthest removed from the plunger. A temperature of approximately 550 C. is maintained at a distance of approximately 8 inches from the bottom of the die, the plunger in this case being inserted from the top. The temperature of the charge decreases at points farther from the bottom of the die. For example, a temperature of 400 may be maintained at a point 20 inches from the bottom of the charge. The plunger is then inserted with a pressure of approximately 20 tons/sq. in. This pressure is maintained for approximately 10 minutes after which the pressure is released and the pressed fuel element removed from the die. Tests have indicated that such an element has a uniform density throughout its length, and that its strength and thermal conductivity are also substantially uniform.

In the pressing of stainless steel a charge of finely divided stainless steel powder is inserted into a die substantially as described with reference to the pressing of uranium. A temperature of approximately 900 C. is maintained at the bottom of the die and the temperature gradient, decreasing at points farther removed from the bottom, is maintained as indicated in the pressing of uranium. A pressure of approximately 70 tons/sq. in. is applied to the charge through a plunger, the upper end of the compressed element being maintained at a temperature of approximately 550. After a short pressing period during which the temperature gradient is maintained, the pressure is released and the pressed element discharged from the die. Uniform density is achieved for elements prepared in this manner.

The method has application to numerous other metals in finely divided form, to mixtures of metals, and to mixtures of metal compounds with finely divided metals.

For example, mixtures of finely divided uranium compounds, such as uranium oxide, with finely divided metals such as stainless steel, aluminum, or zirconium, may be pressed by the gradient temperature pressing procedure to produce elongated elements of substantially uniform density. The ratio of metal to uranium compound in such case is preferably of the order of -50 percent by weight. The element prepared in this manner may be in the ratio of approximately 8 units of length as compared to one unit of width when prepared using single acting dies. Through the use of double acting dies, this ratio may be extended to approximately double that possible through the use of the single acting dies. In this latter case the highest temperature will be at the point fatrhest removed from each die, namely at the center of the charge.

The applications of the elements prepared in this manner are not restricted to those involving the highest density, although this may be desirable for some purposes. Where an element such as a filter is to be prepared from a corrosion-resistant metal powder, such as stainless steel,

nickel, or bronze, it is possible to achieve a uniform porosity of the element by maintaining the temperature gradient to provide the highest temperature at the point farthest removed from the point of pressing. Where such elements are to be prepared in tubular form, a removable. insert or a central extension of the plunger of smaller diameter than that of the die may be used.

Since many embodiments may be made of the subject method, the foregoing is to be interpreted as illustrative only, and not in a restricting sense.

I claim:

1. The method of hot pressing to form an element having a length to width ratio greater than 2 to 1 and a uniform density throughout its length which comprises heating metallic powder within a die to a predetermined temperature gradient throughout its length and applying pressure thereto to compress and sinter the said metallic powder.

2. The method of hot pressing to form an element having a length to width ratio greater than 2 to 1 and a uniform density throughout its length which comprises introducing the entire charge of powdered metallic particles into a die in which it is to be pressed, heating the charge in said die to a predetermined temperature which is non-uniform and gradient throughout its length, the highest temperature being maintained at the point farthest removed from the point at which the pressure is to be applied and compressing the particles within the die.

3. The method of hot pressing to form an element having a length to width ratio greater than 2 to 1 and a uniform density throughout its length which comprises introducing the entire charge of powdered metallic particles into a die in which it is to be pressed, heating the charge in said die to a temperature which is non-uniform and gradient throughout its length and compressing the particles within the die by applying pressure thereto.

No references cited 

1. THE METHOD OF HOT PRESSING TO FORM AN ELEMENT HAVING A LENGTH TO WODTH RATIO GREATER THAN 2 TO 1 AND A UNIFORM DENSITY THROUGHOUT ITS LENGTH WHICH COMPRISES HEATING METALLIC POWDER WITHIN A DIE RO A PREDETERMINED TEMPERATURE GRADIENT THROUGHOUT ITS LENGTH AND APPLYING PRESSURE THERETO COMPRESS AND SINTER THE SAID METALLIC POWDER. 